So I've been looking around on Ebay and I've noticed a lot of listings that say something like "modified nerf blaster" or "custom painted nerf gun" and so on. They also tend to be low-tier blasters too, like retaliators/recons and nitefinders/firestrikes, so i assume these listings are marketed towards cos-players or non nerfers. They tend to be much more costly than their stock counterparts, obviously. Anyone on NH have any experience selling blasters like this? I assume the best way to go about this would be to focus solely on cosmetics, like doing little or no modding to the blaster other than painting it, and using a cheap blaster to maximize profit.

could selling a few purpose-built blasters like this be worthwhile or a waste of time?

It depends entirely on how good you are at cosmetic mods. If you have a background in painting tabletop minis you can probably do pretty well, if you haven't ever painted anything before you'll have to learn. If you can do simple mods like spring upgrades and lock removal that will let you advertise the blasters as improved performance, which would help.

To me, it seems like a wholly junk market. I've seen listings for blasters that looked like they were just painted with no work going for crazy high prices, I've seen carefully modded, integrated, painted blasters going for much less. It seems all brand/name based, if you're known in a community with $$ to spend, you'll make money. If not, you won't.

Generally, our community either doesn't have a lot of money to spend, or can do all the work themselves for less than you'll charge, so in-hobby commissions are rare and require you to be a really respected modder (eg.: Slug level respected).